Fc receptor for shark IgM.

Fc receptors for shark IgM have been demonstrated on shark leukocytes. Measurement of receptor binding required treatment of leukocytes with Cytochalasin D to inhibit phagocytosis. EA rosetting assays were carried out using human erythrocytes coated with shark anti-human antibody. Binding to shark leukocytes was demonstrated to be specific to shark IgM in that affinity purified shark IgM and purified Fc5 mu fragments could block rosette formation, but not shark transferrin, bovine serum albumin or fetal bovine serum. The binding was shown to be saturable and reversible, characteristic of receptor-ligand interaction. Further, it was shown that affinity purified, radioiodinated IgM could also bind Cytochalasin D-treated shark leukocytes in a manner analogous to rosetting. We conclude that Fc receptors appeared early in evolution, and that previous difficulties in demonstrating the Fc mu receptor resulted from non-specific binding associated with phagocytosis.